It is known in the art relating to diesel engines to provide an ignition glow plug having a heated glow tip which extends into the engine combustion chamber or pre-chamber to promote ignition of fuel, especially during starting and low temperature operation. It is also known in internal combustion engines to provide an ion sensor in the combustion chamber which senses the occurrence of combustion events through variations in current flow across a gap through combustion gases in the chamber. The combination of a ceramic glow plug tip combined with an ion sensor for use in a diesel engine has also been proposed.